Happens due to different body chemistry. Sometimes you have to 'play' with the salt-to-water ratio.
Make sure it's Sea Salt , not regular white table salt and that the water is luke-warm. Adjust the Sea Salt level by about 1/4 teaspoon up or down and when it works, you'll know it. Once you know what works for your body, note it for the next time you need to do a SWF.